Such a method and such a bar cutting blade are known from DE 694 05 978 T2 which will be discussed in greater detail further below. To quote from the aforementioned document already at this point, in a bar cutting blade referred to as “profile-sharpened”, the top surface and the two clearance surfaces are ground to restore and resharpen the cutting blade. In this type of bar cutting blade the rake surface is not ground. Such profile-sharpened bar cutting blades may be used for the removal of stock from the outside or concave flank of a tooth slot (outside cutting blade), from the inside or convex flank of a tooth slot (inside cutting blade) and/or from the bottom of the tooth slot (rough cutting blade). By contrast, in a bar cutting blade referred to as “profile-sharpened and form-ground’, sharpening involves grinding the top surface, the two clearance surfaces and the rake surface.
From U.S. Pat. No. 1,667,299 from the year 1928, a form cutting blade is known which is reground only on the rake surface for sharpening. According to the aforementioned definition, a form cuffing blade is a cutting blade known as a “form-ground” cutting blade. Such a form cutting blade or form-ground cutting blade is no bar cutting blade but has a short shank dimensioned as wide as possible in the direction normal to the cutting blade longitudinal axis, because when such a cutting blade is reground stock is ground off in a direction normal to the cutting blade longitudinal axis. The usable profile length for regrinding is determined by the thickness of the form cutting blade normal to the cutting blade longitudinal axis. The profile of the form cutting blade is fixed and cannot be altered by resharpening. The form cutting blade has its two clearance surfaces relieved in an arc-shaped configuration. A variety of pressure angles are obtained by suitably coarsely stepped cutting blades. The shape of the clearance surfaces results necessarily from a selected clearance angle on the top cutting edge and the necessary relief grind. The design constraints with regard to a technologically advantageous clearance and rake angle are accordingly high.
From the article “Spiral- und Hypoidkegelrãder nach dem Spiroflex-Verfahren” (spiral and hypoid bevel gears according to the Spiroflex technique) by Erich Kotthaus, German journal “Werkstatt und Betrieb”, 1967, pages 602-606, the following additional aspects result in this context. In a form cutting blade the tangent of the flank clearance angle must be equal to the tangent of the normal pressure angle times the tangent of the top clearance angle. For grinding, the cutter head with the form cutting blades held therein is mounted on a special sharpening machine, and each of the form cutting blades has its rake surfaces reground individually in the indexing head until the wear marks on the cutting edges are abraded. In order to be able to cut as many teeth as possible per cutting blade, a long usable profile length is required. The space requirements of a cutting blade on the circumference of the cutter head are hence dependent on the profile length on the cutting blade and the space between two adjacent form cutting blades, which is necessary to ensure passage of the grinding wheel necessary for sharpening. The higher the space requirements, the lower the performance of the cutter head because fewer form cutting blades can then be accommodated on the same circumference and, hence, fewer cuts can be taken per unit of time.
While form cutting blades such as the one from the year 1928 cut the complete tooth slot in a single milling pass, machining techniques and quality demands brought about their replacement already a few years after their introduction, substituting a group of form cutting blades having inside and outside cutting blades of the type then used for decades in the manufacture of spiral bevel gears and hypoid gears (see U.S. Pat. No. 2,024,494 from the year 1935 and Gear Handbook by D. W. Dudley, McGraw-Hill, 1962, pages 20-24 and 20-25). U.S. Pat. No. 2,024,494 describes a cutter head with alternating inside and outside cutting blades, with which both bevel gear flanks of a tooth slot can be finished in a single cut using the same machine settings. It is only in a period after 1960 that these form cutting blade groups, which have been in use since 1935 and are comprised of at least one form cutting blade for cutting the concave flank and one form cutting blade for cutting the convex flank, have been replaced in each case by a group of at least two bar cutting blades. The reasons therefor and the advantages achievable with groups of bar cutting blades are clearly described in the aforementioned article “Spiral- und Hypoidkegelrader nach dem Spiroflex-Verfahren”. In this technique, each group of bar cutting blades includes two finishing cutters (one for the concave and one for the convex tooth flank), each having an associated roughing cutter for performing the roughing cut. The combination roughing and finishing cutters are received in a common slot. The mounting of two cutting blades in one slot and the small shank cross-section of these bar cutting blades allow a substantially denser population of cutting blades than would be possible with form cutting blades.
According to the current state of the art, bevel gear milling cutters continue to be used in the form of bar cutting blades. The bar cutting blades used are of high speed steel or carbide. When machining bevel gears in one milling pass, two different cutting edge profile designs of bar cutting blade are used in a cutter head. The bar cutting blades embodying the one design of cutting edge profile machine with the cutting edge arranged on the outside diameter the concave tooth flank (outside cutting blade). Bar cutting blades of this profile design have a special cutting edge geometry leading generally to a positive rake angle. The term positive or negative rake angle is defined, for example, in DIN 6581 of May 1966, page 8, FIG. 13. The bar cutting blades embodying the second design of cutting edge profile machine with the cutting edge arranged on the inside diameter the convex tooth flank (inside cutting blade). Bar cutting blades of this profile design have likewise a special, yet different, cutting edge geometry leading generally also to a positive rake angle. The possibility exists to use one or two roughing cutters in addition to the previously described bar cutting blades.
The number of bar cutting blades adapted to be positioned on a cutter head is limited. Due to the bar cutting blade geometry employed in the prior art, at least two different geometry designs have to be inserted into the cutter head in alternation. In this arrangement only half of the cutting blades can be involved at a time in the generation of the respective tooth flank final geometry.
In the method known as the Oerlikon method for manufacturing bevel gears, the cutter head is equipped with several cutting blade groups each comprised of three bar cutting blades. Each group includes an outside cutter, an inside cutter and a roughing cutter. On each Oerlikon bar cutting blade at least one rake surface and two lateral clearance surfaces at the cutting end are reground. Such cutting blades are designated as triplex flank ground cutting blades or—according to the above definition—as profile-sharpened and additionally form-ground bar cutting blades. Further details relating to the Oerlikon method are contained, for example, in the introductory part of the description of DE 19624685 C1.
In a method according to EP 0 203 085 B1, bar cutting blades of a profile design are used that enable the roughing cutter to be eliminated. Therefore, a group of bar cutting blades includes only two bar cutting blades, which is the reason why more bar cutting blade groups can be accommodated on a cutter head than with the aforementioned Oerlikon method. These bar cutting blades are reground on only two surfaces in the direction of the shank, so that a coating can be applied to the rake surface of these cutting blades which does not necessarily have to be renewed after sharpening, hence prolonging the life between regrinds. Such cutting blades are referred to as duplex flank ground cutting blades or—in accordance with the above definition—as profile-sharpened bar cutting blades.
Cutter heads in which all the bar cutting blades are arranged on a circle such that alternately one bar cutting blade works the concave flank and the next bar cutting blade works the convex flank of one and the same tooth slot, are used in the method referred to as the single indexing manufacturing method. In this method, a tooth slot continues to be machined in one milling pass until the final geometry is obtained. Then an indexing movement to the next tooth slot takes place whereupon this next tooth slot is machined in the next milling pass. By contrast, cutter heads in which the bar cutting blades are arranged in groups find application in the method known as the continuous manufacturing method, in which one cutting blade group machines the convex and the concave flanks in a tooth slot while subsequently the next group of cutting blades enters the next tooth slot where it machines the two tooth flanks. Pertinent details are contained, for example, in the Handbook of Bevel and Hypoid Gears by Hermann J. Stadtfeld, Rochester Institute of Technology, 1993, page 35.
A feature shared by the known methods described in the foregoing is that the machining of a tooth slot invariably requires at least two bar cutting blades whose cutting edge profile is designed to enable the cutting blades jointly to generate a complete final geometry in one milling pass. Furthermore, proper positioning of the individual bar cutting blades of a group of cutting blades in a cutter head is critical and involves an elaborate technique.
A method and a bar cutting blade of the type initially referred to are known from DE 694 05 978 T2 referred to initially. The bar cutting blade is of the profile-sharpened type having its primary or first cutting edge used as outside or inside cutting blade while yet including a second cutting edge on the rake surface in the region of its secondary cutting edge. To obtain the second cutting edge, a slot is produced in the rake surface which forms said second cutting edge whose rake angle differs from the rake angle of the first cutting edge. The second cutting edge cuts a portion of the bottom of the tooth slot as well as a portion of the flank opposite the flank cut by the first cutting edge. The reason for such an elaborate second cutting edge does not become readily apparent from DE 694 05 978 T2. In this document however express reference is made to U.S. Pat. No. 4,575,285. This U.S. patent is based on a prior art in which a cutting blade group is comprised of three cutting blades, i.e., an inside cutting blade, an outside cutting blade and an additional cutting blade for roughing the bottom of the tooth slot. The second cutting edge, which is produced by means of the slot, enables each inside and outside cutting blade to cut not only the associated tooth flank, but also a portion of the opposite flank and a portion of the bottom of the tooth slot. The purpose of this is to omit the need for the roughing cutter and to reduce the cutting blade group to two cutting blades. Hence, two cutting blades per group continue to be necessary in order to be able to generate a tooth slot to its complete final geometry.
Accompanying FIG. 6 illustrates the engagement of a group of cutting blades including an outside cutting blade 60 and an inside cutting blade 66 in a tooth slot 51. The outside cutting blade 60 has a primary cutting edge 61v and a secondary cutting edge 61x opposite the primary cutting edge 61v. The inside cutting blade 66 has a primary cutting edge 67x and a secondary cutting edge 67v opposite the primary cutting edge 67x. With its primary cutting edge 61v, the outside cutting blade 60 machines a first flank 53 of the later tooth slot 51 to provide it with its final geometry. With its secondary cutting edge 61x, this cutting blade machines simultaneously another flank, not shown in FIG. 6, which lies opposite the first flank 53. However, said other flank is not part of the tooth slot 51 with its final geometry but is an intermediate flank serving to facilitate the work of the primary cutting edge 67x of the adjoining inside cutting blade 66 of the group, which cutting blade machines a second flank 54 of the tooth slot 51 opposite the first flank 53 to its final geometry. Accordingly, the second cutting edge provided on the profile-sharpened bar cutting blade according to DE 694 05 978 T2 can at best slightly improve the cutting operation of the secondary cutting edge because it has a rake angle differing from the negative rake angle of the secondary cutting edge and amounting to zero degrees in the embodiment illustrated in DE 694 05 978 T2.
It is an object of the present invention to provide a method and a bar cutting blade of the type initially referred to, which enable the positioning of the cutting blades in a cutter head to be simplified and the machining of bevel gears to be performed with substantially enhanced effectiveness. Furthermore it is an object to indicate a special use of the bar cutting blade.